In amplifying low level signals from a high impedance source such as an electret microphone, the first or pre-amplifier stage of an amplifier may attenuate the incoming signal substantially because of a poor impedance match, may generate excessive noise, and may cause feedback oscillation. Various techniques have been proposed to circumvent these problems, but some of those techniques are difficult to implement, especially in applications that use a low supply voltage, such as the single 1.3 volt power cell usually used in a hearing aid.
A pre-amplifier for a signal from a high source impedance, such as an electret microphone, should have a relatively high input impedance if diminution of the signal is to be avoided. The reason for this is the voltage division that occurs between the signal's source impedance and the input impedance of the amplifier.
Another problem with high source impedances, complicated if a low power supply voltage is used, is noise and other undesirable signals introduced via the power supply conductors. The immediate source of the problem may be a resistance in the power supply or the internal impedance of the power source. This effect is especially troublesome when a power cell nears the end of its life. Unwanted power supply signals can be of various types. Stray pickup of power line hum or of a radio station may occur. The power supply may contribute a signal that, because of high current drawn through the power supply impedance, is equal to or greater than the wanted signal. The circuit may then go into feedback oscillation.
One amplifier that successfully solves some of these problems is disclosed in-Madaffari U.S. Pat. No. 5,083,095 issued Jan. 21, 1992. That amplifier affords improved input impedance and reduced susceptibility to power supply fluctuations, as compared with some prior circuits. But the input amplifiers of that patent still have some problems, particularly as regards noise content in the output signal supplied to succeeding amplifiers. The present invention is aimed at solving those problems.